[SystemZ] Split out comparison elimination into a separate pass

Perhaps predictably, doing comparison elimination on the fly during
SystemZLongBranch turned out to be a bad idea.  The next patches make
use of LOAD AND TEST and BRANCH ON COUNT, both of which require
changes to earlier instructions.

No functionality change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187718 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Add LOAD AND TEST instructions

Just the definitions and MC support.  The next patch uses them for codegen.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187719 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Use LOAD AND TEST to eliminate comparisons against zero


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187720 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Add definitions for BRCT and BRCTG


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187721 91177308-0d34-0410-b5e6-96231b3b80d8
Don't leak passes if added outside of the area determined by Started/Stopped flags.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187722 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Use BRCT and BRCTG to eliminate add-&-compare sequences

This patch just uses a peephole test for "add; compare; branch" sequences
within a single block.  The IR optimizers already convert loops to
decrement-and-branch-on-nonzero form in some cases, so even this
simplistic test triggers many times during a clang bootstrap and
projects/test-suite run.  It looks like there are still cases where we
need to more strongly prefer branches on nonzero though.  E.g. I saw a
case where a loop that started out with a check for 0 ended up with a
check for -1.  I'll try to look at that sometime.

I ended up adding the Reference class because MachineInstr::readsRegister()
doesn't check for subregisters (by design, as far as I could tell).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187723 91177308-0d34-0410-b5e6-96231b3b80d8